A conventional navigation system calculates routes so that a route to be traveled straight is prioritized. In this manner, the navigation system determines a route to a destination at a specified distance far away from the current position. The navigation system calculates routes by prioritizing a route to straight travel a specified distance ahead for the following reason. That is, when the route calculation is performed while traveling, the current position moves during a time from the start to the end of the route calculation. Consequently, the current position after the calculation does not match that before the calculation. In this case, for example, the current position may be determined immediately in front of an intersection immediately after the route guidance starts. A driver may be unexpectedly guided to turn right or left.
For this reason, it is necessary not to calculate the route for right or left turn immediately after the route guidance starts. Therefore, the navigation system prioritizes a straight route to calculate routes to the destination at a specified distance ahead from the current position before the calculation.
However, a distance to be traveled from the start to the end of the route calculation depends on traveling speeds. Accordingly, the distance to be traveled straight is not always ensured optimally. In consideration for this, it may be possible to change a distance to be traveled straight based on a traveling speed when the route calculation starts. However, the route may be unfavorable to a driver even after changing the distance to be traveled straight based on increased or decreased speeds.
For example, let us consider that a vehicle travels a road including many lanes such as four lanes per direction. When the vehicle speed is low, the distance to be traveled straight is changed to be short. Accordingly, it may be impossible to ensure a distance long enough to change lanes.
Alternatively, let us consider that a vehicle travels a road including a few lanes such as one lane per direction. When the vehicle speed is high, the distance to be traveled straight is changed to be long. However, the vehicle can turn right or left without changing the lane. A guidance to go straight is made even though the vehicle can turn right or left immediately, causing a roundabout route to the destination.